The present invention relates to impact printers and more particularly to a novel multiple spring high speed solenoid assembly for use in such impact printers.
Dot matrix printers are typically comprised of a plurality of solenoid driven wires mounted within a movable print head which traverses a paper document. During movement of the print head across the paper document selected solenoids are energized and drive their associated print wires against an inked ribbon and ultimately against the paper document to form dot column patterns at closely spaced intervals across the line of print. In one typical embodiment the print head utilizes seven solenoid driven print wires and successively forms five dot column patterns which collectively form a character, each character being formed within a 5 .times. 7 dot matrix. Selective energization of the solenoids permits the generation of alphabetic and numeric characters, punctuation symbols and the like.
In 132 column printers i.e., printers capable of printing 132 characters per line of print with each character formed within a 5 .times. 7 dot matrix, each individual solenoid may operate as many as 660 times per printed line. In the formation of graphic patterns, each solenoid may be caused to operate up to 792 times per line of print.
The print wires are typically spaced of the order of 0.006 inches from the inked ribbon and paper document. The total distance travelled by a print wire is of the order 0.015 inches. Positioning the forward ends of the print wires a distance less than 0.015 inches from the paper document takes into account some of the force which is absorbed by the ribbon and paper document upon impact.
Conventional printers of the above mentioned category are capable of printing at the rate of 330 characters per second and 125 lines per minute (for lines of 132 character length). In order to achieve these print speeds, the print wire must be capable of being accelerated from a rest position to a velocity sufficient to form a dot on the original document and typically five carbon copies and return to its rest position in less than one millisecond. It has been found to be impractical to obtain faster operating speeds using conventional present day solenoid designs.